Link is a toggle; hit it once to Unlink, then reselect and hit it again to Link. I've mapped it to Ctrl+L.
Group and Ungroup are two separate commands, weirdly enough. Ctrl+G is Group, Shift+Ctrl+G is Ungroup.
I've made a feature request that Group and Ungroup act as a toggle, just like Link. It doesn't make sense to waste a keyboard shortcut.